Prior to the present invention, dual-circuit hydraulic brake systems have been used and taught in the prior art. See, for example, VISITA-PAPER 86 51 40, 1986, page 2, 265. The hydraulic brake system taught in this reference has a hydrostatic circuit which is fed from a master cylinder as a function of the force being exerted by a vehicle operator's foot. There is also provided a servo circuit in this hydraulic brake system. The servo circuit is supplied with hydraulic fluid from a hydraulic pressure source. Such hydraulic pressure source includes both a pump and a high pressure reservoir.
In this prior art hydraulic brake system, hydraulic fluid is communicated to the wheel brake cylinders from both the hydrostatic circuit and the servo circuit, during a brake application, by way of two servo cylinders. One of these two servo cylinders is associated with each wheel on the front axle. The hydrostatic circuit fed by the master cylinder is connected to an annular receptacle portion of such servo cylinder. This annular receptacle increases in size as the braking force being applied is increased. The servo circuit, that is, the hydraulic pressure source, is connected to a cylinder chamber disposed in the servo cylinder.
This hydraulic brake system requires the two servo cylinders to perform several distinct functions. On the one hand, for example, the servo cylinders separate the hydrostatic circuit and the servo circuit from one another. The separation of these circuits is necessary for safety reasons. On the other hand, these servo cylinders match the different pressure levels of the hydrostatic circuit and the servo circuit to one another. This matching of the different pressure levels present in these circuits is accomplished in this prior art hydraulic brake system by means of suitable translation ratios or by piston surfaces. Such servo cylinders also make possible the pressurization of the wheel brakes controlled by a respective one of such servo cylinders by the servo circuit pressure on the one hand and, simultaneously, by the master cylinder pressure on the other hand.
For the realization of an anti-locking function in the hydraulic brake system taught in the prior art, the servo cylinders are preceded by certain control valves positioned on the hydrodynamic booster side. These control valves can be magnetically activated by means of an electrical control signal supplied to them by an anti-lock electronic system. The locking of such wheel brakes is prevented in a manner which is well known in the braking art. In particular, in emergency braking conditions, it is of extreme importance to prevent such wheel brakes from locking. The anti-lock system is preferably designed as an individual regulation (IR) system. In an IR anti-lock brake system, each of the wheel brakes can be regulated individually.
As is known in the prior art, the hydraulic brake systems, as described above, can also be supplemented by two additional solenoid-controlled valves. Such additional solenoid-controlled valves are commonly known as separation valves. By means of these separation valves, in the case of a regulated or anti-lock braking application, the master cylinder can be separated from the servo cylinders by means of an appropriate control signal from the anti-lock electronic system. In this manner, undesirable feedback or pumping-type movement from the regulating anti-lock protection system to the brake pedal can be prevented. In addition to preventing such feedback, a disruptive influence on the regulation of the anti-lock braking system from the master cylinder is eliminated. Further, the annular receptacle portion of the servo cylinder is thereby connected directly to the pressure medium compensation reservoir of the hydraulic brake system. Such direct connection is necessary so that the brake pressure in the individual regulation cycles can be reduced down to a zero value if necessary.
This prior art dual-circuit hydraulic brake system design, however, has the disadvantage that the hydrostatic circuit can no longer be considered a closed system in the event the servo circuit is involved in an anti-lock brake operation. This circumstance presents the danger that the master cylinder can run at no load in the case of a defect or malfunction of one or both of the above-described supplementary solenoid-controlled valves.